With the current computing and information revolution, portable electronic devices such as cellular telephones, personal digital assistants (PDAs), digital pagers and wireless email devices, are becoming very common.
These portable devices are typically powered by internal batteries which must be recharged periodically by an external power source, using a battery charger. Battery chargers generally receive power from a standard AC electrical outlet and convert the AC power into a low DC voltage for recharging a battery.
The battery chargers of these portable devices also generally employ a “battery charge controller” to manage the charging of the battery. Such battery charge controllers offer functionality such as:                regulating the voltage and current levels to the rechargeable battery;        providing status signals to the main processor of the portable device, or operating one or more status LEDs (light emitting diodes);        providing protection circuits such as overcurrent, undervoltage, and overtemperature protection; and        shutting themselves off when the charging source has been removed, to minimize battery drain.Unfortunately, most of these battery charge controllers are designed to draw from a high capacity power supply with a steady voltage that will not sag appreciably under its current demands. This is a problem when one attempts to use a power supply with limited capacity or when the power is delivered via unknown length and gauge of power wires (the resistance of such power wires cause a voltage drop which aggravates the problem). Some computer data buses such as USB (universal serial bus) buses can be used to provide power to external devices, but while such power supplies are very convenient, they have limited capacity and are delivered via standard USB cables whose power wires could vary in length and gauge (anywhere from 20 AWG–28 AWG, for example).        
The majority of personal computers (PCs) and laptop computers available today, are provided with one or more USB ports as standard components. USB ports are designed to support data communication at speeds of 12 megabits and 1.5 megabits per second (USB 2.0 provides for up to 480 megabits per second), support PnP (Plug and Play) installation software, and support hot plugging (i.e. devices can be connected and disconnected while the PC is running). Thus, USB ports are often used as interfaces to connect keyboards, mouses, game controllers, printers and scanners to PCs.
As well, USB ports are able to supply limited power to a connected device. The standard USB specification requires that “high-power” USB ports be operable to provide a supply voltage of 4.75–5.25 VDC and supply a maximum current of at least 500 mA (five units). The specification for “low-power” USB ports requires a supply voltage of 4.40–5.25 VDC and current of 100 mA (one unit).
USB ports would seem to be a very logical choice as a power supply for portable devices for a number of reasons. To begin with, USB ports supply a low DC voltage supply which is often very close to, or just above, the voltage of the battery being charged (many portable devices having battery voltages in the range of 2.5–4.5 VDC.) As well, many portable devices may be operable to upload and download data or software, to and from a personal computer or a laptop computer (often referred to as “syncing.”) Thus, may portable devices are supplied with docking cradles as shown in the system diagram of FIG. 1. This is quite a straightforward system as the docking cradle 10 is connected to a USB port 12 of a personal computer (PC) 14, via a simple USB cable and connectors 16. The mobile device 18 need only be placed into the docking cradle 10 and an electronic connection to the (PC) 14, is made.
If the USB port 12 has sufficient power, it makes much more sense to use the USB port 12 to supply charging power to the mobile device 18, rather than using a separate AC charger. For example:                a USB power supply will have less electrical noise than an AC charger, unless the AC charger incorporates large DC capacitors or inductors;        an AC charger requires either a heavy transformer or an expensive switching power supply current, neither of which would be required if USB power is used;        in the USB power supply implementation, the cable and connectors 16 used to connect the docking cradle 10 to the PC 14 could be used to carry both power and data, so no extra physical components would be required at all. In contrast, an AC power supply would have to be provided as a separate physical component, and        a given AC power supply may require 120VAC or 240VAC as an input, and may provide 3, 4.5, 6, 7.5 or 9 VDC out, with one of a large number of different possible connectors and polarities. A traveler who forgets an AC power supply at home, may not be able to find a replacement.        In contrast, the USB standard is widely accepted, so that a traveller whose mobile device is equipped with a USB connector will have a much greater chance of finding a charging source.        
Unfortunately, USB ports can only provide limited power, while typical battery charge controllers are designed to receive a steady, high capacity power supply (that is, the input voltage at the battery charge controller is at or near its designed value, and does not drop as charging current increases.) The problem becomes clear when considering the block diagram of FIG. 2. The components of FIG. 2 are the same as those of FIG. 1, specifically, a docking cradle 10 powered from a USB port 12 of a PC 14, via cable and connectors 16, and feeding a portable device 18 resting in the cradle 10. From this presentation, it is clear that the voltage output from the USB port 12, VUBS, will drop as it crosses the cable and connectors 16, due to its resistance, RCABLE. If either the cable resistance (RCABLE) or the current drawn (ICABLE) is too great, the voltage arriving at the battery charge controller in the mobile device 18, may be too low. This low voltage will cause many standard battery charge controllers to either shut down or to oscillate and fail to charge the battery in the portable device 18 efficiently.
True, new dedicated battery charge controllers could be developed which are operable with the limited USB power supply and resistance of the cable and connector system, but that would be an expensive and complicated solution. Such a design would become even more complicated to be compatible with both computer data bus power supplies and other power sources, such as AC power supplies.
There is therefore a need for a method and apparatus which allows standard battery charge controllers to be supplied with power from standard computer data busses such as USB ports. This design must be provided with consideration for the cost of electrical components, the limited physical board area in portable devices, the reliability and the complexity of the design. It is also desirable that this method and apparatus be operable with both computer data bus power supplies, and other power sources such as AC power supplies.